1. Technical Field
The present invention relates to a hybrid working vehicle having an engine and an electric motor for driving a drive wheel and a power take-off (PTO) shaft.
2. Related Art
Recently, as disclosed by JP 2007-269072 A, a hybrid drive system including an engine, a generator driven by the engine, and electric motors driven by electric power generated by the generator is used by working vehicles, e.g., construction machines and agricultural machines, as well as motorcars. A working vehicle using this hybrid drive system selects either the engine power or the electric motor power so as to drive front and rear drive wheels and its PTO shaft, thereby achieving various driving modes in travel of the vehicle and in work by drive of the PTO shaft.
In this regard, the electric motors can change their output rotary speeds steplessly (continuously). In other words, the output rotary speeds of the electric motors do not change sharply. On the other hand, the output rotary speed of the engine is almost kept constant during travel or work of the vehicle. Therefore, to change the traveling speed of the vehicle based on the engine power, a gear transmission on a traveling drive train for transmitting the engine power to drive wheels must be gear-shifted. Further, to change the drive speed of the PTO shaft based on the engine power, a gear transmission on a PTO drive train for transmitting the engine power to the PTO shaft must be gear-shifted. The gearshift of these gear transmissions is stepwise. For example, during the gearshift from a high speed stage to a low speed stage, the traveling speed of the vehicle reduces considerably sharply. Accordingly, in a comparison between the engine-powered traveling or working state and the electric motor powered traveling or working state, there is a great difference in speed changing pattern during travel or work of the vehicle or in an operator's feeling of operation for speed change of traveling or work of the vehicle. Therefore, the traveling speed of the vehicle and the drive speed of the working implement of the vehicle are desired to be changed steplessly and smoothly regardless of whether the driving of the drive wheels or the working implement is caused by the engine power or the electric motor power.
Further, the output power of the electric motor for traveling of a vehicle is inputted to downstream portions of the traveling drive train while the engine power is canceled. On the other hand, the output power of the electric motor for PTO driving is inputted to an intermediate portion of the PTO drive train while the engine power is canceled. Therefore, an electric motor for driving rear wheels, an electric motor for driving front wheels and an electric motor for PTO driving are necessary, thereby increasing the number of parts and costs and inhibiting minimization of the vehicle.
Further, as disclosed by the above-mentioned prior art document, the convenient hybrid working vehicle alternately transmits either the engine output power or the electric motor output power to the drive wheels or the PTO shaft. In other words, since the electric motor power cannot be combined with the engine power, the function of the electric motor for assisting the engine in torque cannot be effectively exerted during starting or acceleration of the vehicle by use of the engine power or during high loaded work of the working implement by use of the engine power, thereby increasing loss of torque and energy. In this way, the problem of the conventional hybrid working vehicle is its insufficient effect in energy saving and in leveling the engine load.